Molecular sieve zeolites have demonstrated substantial utility in a wide variety of adsorptive, catalytic and ion exchange processes. Owing to engineering considerations, the physical form in which the zeolites are employed may exert a profound influence on the efficiency of a given process. While considerable effort in the prior art has been devoted to the preparation of various zeolite bodies in forms suitable for fixed bed or fluidized bed applications, no such reports of massive bodies of Maximum Aluminum X-type zeolite are known. Maximum Aluminum X-type zeolite is the only known wide pored (about 8.ANG.) zeolite available in this form possessing an idealized 1:1 silica/alumina ratio. For this and perhaps other reasons, it demonstrates a variety of unique and desirable properties.
U.S. Pat. No. 3,119,660 teaches the preparation of massive bodies of molecular sieve zeolites by the in situ conversion of meta kaolin or meta kaolin/silica aggregates. Using these meta kaolin aggregates as the silica-alumina source, standard chemical formulations are shown (as described in U.S. Pat. No. 3,114,603) to facilitate the conversion of meta kaolin to massive bodies of high purity (&gt;90%) zeolite A. U.S. Pat. No. 3,119,660 further teaches that zeolites with elevated Si/Al ratios may be prepared by the conversion of meta kaolin aggregates whose silica content has been fortified by the addition of a supplemental silica source. The preparation of the faujasite type zeolites X and Y are specifically demonstrated using these techniques.
An alternate method of preparation of massive bodies of zeolite is disclosed in U.S. Pat. No. 3,359,068 wherein preformed, calcined aggregates derived from a silica gel are reacted in situ with appropriate alkali-metal-aluminate solutions. Such compositions are said to demonstrate superior adsorptive properties when compared with their classically prepared counterparts. The analogous technique of the in situ conversion of preformed aggregates derived from an alumina hydrosol by contact with the appropriate alkali-metal-silicate is taught by U.S. Pat. No. 3,386,802.
Other variations on the preparation of selfbound zeolite aggregates included the preparation of classically bound zeolite extrusions wherein the binders are further converted to zeolite. For example Zeolite A (as reported in U.S. Pat. No. 3,395,989) or Zeolite X (as reported in U.S. Pat. No. 3,906,076) may be bound with meta kaolin and this binding phase subsequently converted to zeolite by known methods.
The prior art relating to the preparation of faujasites has centered on the elevation of the Si/Al ratio by either imparting aluminum deficiency on clays by techniques such as acid leaching, as taught in U.S. Pat. No. 3,733,391, or using high silica hydrols, as taught in U.S. Pat. No. 3,459,501 as the starting reactants.
In no case is the preparation of Maximum Aluminum X-type zeolite in the form of massive bodies disclosed.
Zeolites span a wide range of aluminum contents from the maximum allowable, according to the rule of Lowenstein (i.e. Si/Al =1.0), as in the case of Zeolite A, to virtually zero as in the case of Silicalite.
Faujasite-type zeolites are defined as those zeolites with a framework topography resembling the mineral faujasite. Such species are characterized by a relatively open zeolite framework with large (about 8.ANG.) pores and high (nearly 50%) void volumes. Synthetic faujasites are generally subdivided into zeolites X and Y by virtue of their silica content; zeolite Y being defined as Si/Al.gtoreq.1.5, zeolite X being defined as Si/Al&lt;1.5. While zeolites X and Y represent a relatively smooth continuum of Si/Al ratio of the faujasite structure over a broad range, a discontinuity appears when the Si/Al ratio is lowered to about 1.2. Extrapolation of standard synthesis techniques for Zeolite X will not produce faujasites below this boundary. As reviewed in U.S. Pat. No. 4,289,740 and 4,407,782, studies of the variability of Si/Al ratios of faujasites terminate at this lower boundary and X zeolite with increased alumina (e.g., Si/Al =about 1) has been referred to as "idealized".
Such a 1:1 Si/Al faujasite may not only be viewed as idealized from the perspective of its unavailability, but also from a structural perspective. Such a species would demonstrate perfect uniformity in alternating silica-alumina groups with concomitant perfect uniformity in pore geometries and high homogeneity in adsorption and catalytic sites.
Little mention of the preparation of an "idealized" 1:1 Si/Al faujasite in the powder form is to be found in the prior art, and none is known relating to the preparation of such a species in the form of massive bodies.
In U.S. Pat. No. 4,289,740 and 4,407,782, a zeolite is prepared by aging and then reacting various alumino-silicate mixtures at extreme pressure, typically 50,000 psi. This zeolite, called HP, is claimed to violate the rule of Lowenstein and demonstrate a Si/Al&lt;1 ratio with a faujasite type structure. However, the x-ray unit cell constants (typically 25.00.ANG.-25.05.ANG.) listed in these patents suggest that, in fact, zeolite HP was similar to a mixed Na/K X-type zeolite earlier reported in East German Patent Nos. 43,221 and 58,957. While not listing a unit cell constant, the X-ray d-spacings given by the earlier East German patents indicate a unit cell constant of 25.10.ANG.-25.15.ANG.. In reduced silica faujasite systems, conversion of Na/K forms to the pure Na form (as in the case of HP), results in a reduction of a.sub.o (the unit cell constant) by about 0.10A units, to exactly that reported for the "new" zeolite HP. The East German patents present gross empirical data on the composition of the claimed "Na/K type zeolite", indicating an Si/Al ratio of about 1; however, such estimates are subject to distortion due to inclusion of unreacted silica and/or alumina in the product.
The only clear claim of the preparation of relatively pure powdered "idealized" 1:1 Si/Al faujasite appears in British Pat. No. 1,580,928. In this British patent, a method of preparation of this "reduced silica X-type zeolite" is described wherein alumino-silicate gels are subjected to a multi-day aging period at low temperature, ideally 40.degree. C., followed by crystallization at 60.degree. C-100.degree. C. at ambient pressure. Attempts to crystallize immediately, without aging as reported in the East German works, failed to give the desired product. The utility of this material as a water softening agent has been demonstrated. In none of the above cases was the preparation of massive bodies of Maximum Alumina X-type zeolite addressed.
With an inate 1:1 Si/Al ratio and known propensity to form zeolite A, meta kaolin would seem a logical base material for the formation of Maximum Aluminum X-type zeolite. Perhaps owing to the discontinuity of Si/Al ratios when the Si/Al ratios of faujasites is lowered to about 1.2 and the inability of extrapolation of standard X-type zeolite reaction mixtures below this ratio, no prior art on the formation of an idealized 1:1 Si/Al faujasite from meta kaolin is known to exist. Using a kaolin based material, in fact U.S. Pat. No. 3,119,660 states "In producing preformed silica rich zeolites such as X and Y . . . additional silica over and above the amount of silica supplied by the reactive kaolin must be provided."